1. Field of the Invention
The present invention relates in general to a hydraulically operated brake system of a motor vehicle, and more particularly to an improvement in accuracy of control of the brake pressures for the vehicle wheels by an anti-lock brake pressure control means of such brake system.
2. Discussion of the Related Art
Such anti-lock brake pressure control means of a hydraulic brake system of a motor vehicle is adapted to maintain the slip amount or slip ratio of each wheel of the vehicle within an optimum range during braking of the vehicle, for thereby shortening a required braking distance of the vehicle while maintaining high running stability of the vehicle under braking. To this end, the anti-lock type hydraulic brake system generally includes wheel speed detecting means for detecting the rotating speeds of the front right and left wheels and the rear right and left wheels of the vehicle, hydraulic brakes operated by hydraulic brake pressures applied thereto to lower the speeds of the respective wheels, and an anti-lock pressure regulating device for regulating the brake pressures on the basis of the wheel speeds detected by the wheel speed detecting means, so as to prevent excessive slipping of the wheels.
Usually, the brake pressure for each wheel is regulated to maintain a difference between the detected wheel speed and an actual running speed of the vehicle within an optimum range for thereby maintaining the slip amount or ratio of the wheel within the optimum range. According to this arrangement, it is necessary to detect the vehicle speed as well as the wheel speeds.
As means for detecting the vehicle speed, it is known to use a Doppler-effect type sensor for detecting the ground speed of the vehicle, namely, the speed of the vehicle with respect to the ground or road surface on which the vehicle is running. Since such Doppler-effect type speed sensor is generally expensive, however, the vehicle speed is commonly estimated from the wheel speeds, as disclosed in JP-A-60-50060. Described more specifically, a highest one of the detected speeds of the four wheels is used as the vehicle speed, that is, as an estimated vehicle speed, in view of the fact that the highest wheel speed is closest to the actual vehicle speed. The thus obtained estimated vehicle speed may be smoothed as needed before it is used as the vehicle speed.
But, the estimated vehicle speed may be considerably lower than the actual vehicle speed if the amount of slip of the highest-speed wheel is excessively large during braking of the vehicle. In other words, even the highest-speed wheel may more or less slip on the road surface, due to an excessively high brake force applied thereto, which acts to lower the speed of the highest-speed wheel. When the highest-speed wheel has a large amount of slip, therefore, the slip amounts of all the four wheels would become excessive, leading to inaccuracy of anti-lock regulation of the brake pressures, if the brake pressures applied to the hydraulically operated brakes were regulated on the basis of the estimated vehicle speed, that is, the highest one of the speeds of the four wheels.